The regulation of cysteine biosynthesis in S. typhimurium involves a complex interacting system of feedback inhibition and repression. The control of gene expression is positive rather than negative as is the case in the well-defined lac operon. Regulatory elements include one or more of the biosynthetic enzymes, an unidentified regulatory gene product, cysteine itself, and several metabolic intermediates. By means of careful genetic and biochemical analyses we shall attempt to define the manner in which these many substances interact to control the rate of cysteine biosynthesis. Efforts will be made to isolate and characterize the as yet unidentified cys B gene regulatory product. Studies on the inducible, degradative enzyme, cysteine desulfhydrase, will complement our work on the biosynthetic pathway.